In the manufacture of concrete structures, the concrete is usually cast using a form. The cast concrete takes the shape of that form. The wet concrete is poured into or against the concrete form and, upon setting of the concrete and removal of the form, the newly-exposed concrete surface is a reverse impression of the inner surface of the form. In the case of wooden forms, the concrete takes the appearance of the wood grain; and in the case of forms involving seamed form members, the concrete shows any seams which have not been sufficiently masked.
In addition, air is often added to a concrete mix and water is often added in excess of the amount required for hydration. Such air and water are useful to render the mix flowable and to facilitate handling and pouring. However, the excess water, if left undrained, results in concrete having a weak surface and, the air, if not removed, results in surface pores as large as 0.1 to 3 cm, which pores leave an uneven surface open to the effects of dirt and erosion by the freeze-thaw cycles of water.
One solution to eliminating seams and surface pores in the cast concrete is to tension a porous fabric over the inner surface of the form prior to casting the concrete. (The porous fabric is usually referred to as a formliner). Upon removal of the form, a relatively smooth surface is left on the cast concrete. Although this method and form have been disclosed in the applicant's co-owned, co-pending U.S. patent applications (U.S. Ser. No. 07/625,721 and U.S. Ser. No. 07/698,991), there is still a need for an apparatus and method that easily and unifomly tensions the porous fabric over the form. In these co-pending applications, the fabric is preferably tensioned by springs or elastomeric grippers that pull the fabric around the edges of the form.
When using these prior art tensioning methods, it is often difficult to obtain uniform bi-directional tension across the fabric and it is often difficult and time consuming to affix the fabric to the form, especially when out in the field where low temperature and wind may be present. Clearly, what is needed is an apparatus and method for attaching and bi-directionally tensioning a porous fabric to a form that don't have the deficiencies inherent in the prior art. Specifically, the apparatus and method should allow fixation and tensioning of the fabric to the form in a shop thereby reducing the amount of work necessary in the field. Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the attached drawing and to the detailed description of the invention which hereinafter follows.